Quantcast

Bubble party--acrylic canopy transparencies

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

BoKu

Pundit
HBA Supporter
Joined
Aug 15, 2013
Messages
2,796
Location
Western US
FYI, our next Akaflieg build session will be a bubble party--we're going to learn the art and technique of stretch-forming acrylic sailplane canopy transparencies over male plugs. I'll collect the plugs for HP-14, HP-18, and HP-24, and we'll pull a few bubbles and see what we get. If you have a canopy plug, bring it and we'll see if we can form to it.

Scheduling is up in the air; it depends on a lot of factors, including the availability of the acrylic forming expert I'm bringing in to lead us. But probably sometime in the early weeks of January. Location isn't locked down either, but it will be either at our shop in Arnold CA or at our logistics facility in Montague CA.

Plan on pandemic precautions: Masks or respirators (not those goofy clear plastic face shields) at all times, daily temperature checks, and as much personal space margin as we can manage given the work we're doing. Unlike typical Akaflieg sessions, we will will not have group accommodations, but there will be hotels nearby.

Oh- and one thing we probably won't be doing is attempting to free-blow canopy bubbles. Sure, it's a perfectly valid method, but it just isn't what works for what I'm doing.

Email me bob@hpaircraft.com if you're interested, and I'll start a list.
 

paraplane

Active Member
Joined
Jul 22, 2014
Messages
44
Location
California
Is this in association with the Akaflieg comprised of Students of Cal-Poly SLO? I know they had worked with you in the past on a few things. I don't know what I'll be doing around January and can't commit to being there but if available I would like to be there.
 

BoKu

Pundit
HBA Supporter
Joined
Aug 15, 2013
Messages
2,796
Location
Western US
Is this in association with the Akaflieg comprised of Students of Cal-Poly SLO? I know they had worked with you in the past on a few things. I don't know what I'll be doing around January and can't commit to being there but if available I would like to be there.
No, this is completely separate from Akaflieg SLO. This is one of the HP Aircraft "Akaflieg" (in quotes) build sessions that we regularly convene to make parts for HP-24 (and now HP-26) kit builders. But it would be nice if some of the Cal Poly students could make it, it's always great having them around.
 

kubark42

Active Member
Joined
Oct 19, 2020
Messages
41
A bit of thread drift, but what criteria did you use for choosing the canopy material and thickness? This is both one of the easiest places to save weight through a judicious choice, and one of the easiest places to make false economies by not considering the edge cases.
 

BoKu

Pundit
HBA Supporter
Joined
Aug 15, 2013
Messages
2,796
Location
Western US
A bit of thread drift, but what criteria did you use for choosing the canopy material and thickness?...
I generally just copy prevailing practice. Most modern sailplane canopy transparencies are 1/8" or 3mm, so that's what I use for the HP-24. The HP-18 is the same. The earlier HPs had short center canopies with thin (~0.090") material, and when I had a few of those made that's what I used.

As for material, it's all acrylic. I'm told polycarbonates are much harder (read: expensive) to form, so I use acrylic unless someone wants to pay for the extra trouble.
 

kubark42

Active Member
Joined
Oct 19, 2020
Messages
41
Polycarbonates also embrittle under UV and shatter, which is why we never use them in cruising sailboats.

On my trimaran, I switched from using pure acrylic to "impact modified acrylic" (aka rubberized acrylic) for a 60cm x 90cm companionway hatch, and was very satisfied with the results. Because I was able to use a thinner sheet, it wound up being several pounds lighter, and cheaper to boot.

While the overall strength and rigidity wasn't as high, the impact resistance was significantly higher and because it weighed so much less the hatch was less likely to break itself if it ever slammed shut under its own weight. (Frequently it seems it doesn't matter if we're sailing boats or sailing planes, the problems repeat themselves.)

That being said, I am less certain of the benefit of reinventing the wheel in the context of a sailplane. I imagine that the EU glider manufacturers have left no stone unturned for easy, cheap ways to reduce weight.
 
Last edited:

BBerson

Light Plane Philosopher
HBA Supporter
Joined
Dec 16, 2007
Messages
14,015
Location
Port Townsend WA
Acrylic can be extruded (cheap) or cast (expensive but best optics).
Cast is hard to find.
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
5,365
Location
US
Polycarbonate is more resistant to impact than acrylic (but is more expensive, harder to form, and scratches more easily.)

Video of polycarbonate vs acrylic motorcycle windscreen taking impact abuse.
Polycarbonate (Lexan) vs Acrylic (Lucite) - Colorado Plastics

In a high airspeed birdstrike I suppose there could be a difference, but in a sailplane and with a relatively high grazing angle it's probably not a significant issue. Acrylic is a lot more popular in GA bubble canopies, and seems to perform very well.
 
Last edited:

kubark42

Active Member
Joined
Oct 19, 2020
Messages
41
Polycarbonate is more resistant to impact than acrylic (but is more expensive, harder to form, and scratches more easily.)

Video of polycarbonate vs acrylic motorcycle windscreen taking impact abuse.
Polycarbonate (Lexan) vs Acrylic (Lucite) - Colorado Plastics

In a high airspeed birdstrike I suppose there could be a difference, but in sailplane and with a relatively high grazing angle it's probably not a significant issue.
Thanks for the link. I did some reading and it turns out they use them in racecars as well, which have the same oblique impact angles, but with much more risk of sharp impact. I think we can conclude that if you keep your canopy out of the sun (and on sailplanes who doesn't?), polycarbonate will keep its impact resistance properties.

I wonder how absurdly thin you can make your canopy if you did it out of polycarbonate. It might be harder to form it to an equivalent thickness of acrylic, but if you get a sheet which is half as thick I'm thinking it will wind up being easier to form. Certainly you'll need a lot less heat, meaning it will be a lot easier to control the heat.
 

BoKu

Pundit
HBA Supporter
Joined
Aug 15, 2013
Messages
2,796
Location
Western US
...I wonder how absurdly thin you can make your canopy if you did it out of polycarbonate. It might be harder to form it to an equivalent thickness of acrylic, but if you get a sheet which is half as thick I'm thinking it will wind up being easier to form. Certainly you'll need a lot less heat, meaning it will be a lot easier to control the heat.
Let's be careful here. Lexan might be stronger than acrylic, and is certainly tougher, but it is also a lot less stiff. And like many things in aeronautics, for canopies stiffness is more important than strength.

Back when I had an HP-11, I replaced its 1/16" acrylic forward canopy transparency with a piece of Lexan, and stuffed the old acrylic piece under my sports car as an oil drip tray.

I found that the Lexan was a lot more limber than the acrylic when I was flying through hail and the forward canopy would shimmer like a soap bubble with each hailstone hit--and these were not unusually large hailstones, just the usual pea-sized ones. As I sped out of the hail, I kept a hand on the inside of the forward canopy to hold it still.

After about four years, the Lexan was so brown from sun exposure that I had trouble seeing through it when flying into the sun. So I dug out the old acrylic sheet from under the sports car, wiped off the oil, polished it up, and reinstalled it. It was on the glider when I sold it about eight years later.
 

Deuelly

Well-Known Member
Log Member
Joined
Nov 6, 2012
Messages
245
Location
Marshall, MN
I also like how in a turnover situation acrylic will break when you hit it instead of bouncing the tool back in your face.

Brandon
 
  • Like
Reactions: BJC

kubark42

Active Member
Joined
Oct 19, 2020
Messages
41
Let's be careful here. Lexan might be stronger than acrylic, and is certainly tougher, but it is also a lot less stiff. And like many things in aeronautics, for canopies stiffness is more important than strength.

Back when I had an HP-11, I replaced its 1/16" acrylic forward canopy transparency with a piece of Lexan, and stuffed the old acrylic piece under my sports car as an oil drip tray.

I found that the Lexan was a lot more limber than the acrylic when I was flying through hail and the forward canopy would shimmer like a soap bubble with each hailstone hit--and these were not unusually large hailstones, just the usual pea-sized ones. As I sped out of the hail, I kept a hand on the inside of the forward canopy to hold it still.

After about four years, the Lexan was so brown from sun exposure that I had trouble seeing through it when flying into the sun. So I dug out the old acrylic sheet from under the sports car, wiped off the oil, polished it up, and reinstalled it. It was on the glider when I sold it about eight years later.
What a great data point! I oftentimes find in structural design that you optimize for stiffness you get strength for free and vice versa. So it sounds like canopy design is in this former category, where in order to be stiff enough, it winds up being stronger than necessary. And in that context, moving to the 40% stiffer acrylic is a win because it trades off unused strength for useful rigidity.

The only way to really solve for rigidity, then, is to engineer a support structure. And that will either be not optically clear, or not affordable.

Well, thanks for sharing this story. It was exactly what I was looking for.

--------

P.S. Out of curiosity, for how long was the Lexan in the sun? I have read things that suggest polycarbonate starts to yellow after ~1000hrs of exposure, which is basically 50 days in the tropics. This is why it's terrible for sailboats, because 50 days goes by in a heartbeat. But in a glider where it hardly sees more hours of sun than it does flight hours, 1000hrs is more than many will fly in a lifetime.
 
Last edited:

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
5,365
Location
US
Polycarbonate sheet is available with coatings on one or both sides that make them highly UV resistant. For example LEXAN XL102UV is transparent and used in skylights, warranted for 10 years of exposure against discoloration or breakage. All modern military bubble canopies are made of polycarbonate, and some of these jets stay outside in the sun a lot.
But, this thread is supposed to be an announcement about a get together to make an acrylic canopy for a sailplane, and that sounds like a lot of fun.
 

ToddK

Well-Known Member
HBA Supporter
Joined
Jan 13, 2016
Messages
393
Location
The Real Texas
Polycarbonate sheet is available with coatings on one or both sides that make them highly UV resistant. For example LEXAN XL102UV is transparent and used in skylights, warranted for 10 years of exposure against discoloration or breakage. All modern military bubble canopies are made of polycarbonate, and some of these jets stay outside in the sun a lot.
But, this thread is supposed to be an announcement about a get together to make an acrylic canopy for a sailplane, and that sounds like a lot of fun.
Any word on polycarbonate that is scratch resistant?

I have used polycarb before on airplanes because of its toughness, and resistance to shattering. One was replacing an old acrylic skylight that shattered and few off on final nearly killing the pilot and passenger. The install was pretty easy, but it ended up with a few scratches that would not have been a problem with acrylic.
 

BrianW

Well-Known Member
Joined
Jul 2, 2018
Messages
130
Location
Altus SW Oklahoma
Any word on polycarbonate that is scratch resistant?

I have used polycarb before on airplanes because of its toughness, and resistance to shattering. One was replacing an old acrylic skylight that shattered and few off on final nearly killing the pilot and passenger. The install was pretty easy, but it ended up with a few scratches that would not have been a problem with acrylic.
Fact-Checking is a disease with me:
Polycarbonate sheet Hardness R 118
Tensile ultimate 9500 psi
Softening temp 152 degC

Acrylic sheet Hardness Rockwell M 93 (about the same as Rockwell R 118)
Tensile ultimate 10000-11700 psi
Softening 115 deg C


Cannot put my finger on the reason for scratch resistance differences?
 
Top